Field of the Invention
The present invention relates to rigid and semirigid thermoplastic structures with multiple coextruded polymeric layers incorporating an inner vinylidene chloride copolymer layer, and to the use of these structures for the manufacture of packages.
Description of the Background
It is well known to improve the gas and vapour impermeability of various packages made of bulk thermoplastic polymers by combining them with a vinylidene chloride copolymer in a structure with multiple polymeric layers. Coextrusion, that is to say simultaneous extrusion through a single die, is an application method which is particularly suitable for combining various layers of thermoplastic polymers in a single operation. However, owing to the relative thermal instability of vinylidene chloride copolymers, the latter usually form an inner layer of the coextruded multilayer structures.
In Belgian Patent 777,943 (the Dow Chemical Co.), a description is given of rigid thermoplastic structures with five coextruded polymeric layers comprising a central vinylidene chloride polymer layer, outer layers of polyvinyl chloride and intermediate adhesive layers of a copolymer of ethylene and vinyl acetate.
It has now been found that the delamination resistance of multilayer structures of the prior art is not optimal and, furthermore, that it deteriorates considerably after a period of residence in boiling water.
SUMMARY OF THE INVENTION
The present invention aims to provide new rigid and semirigid thermoplastic structures with multiple coextruded polymeric layers incorporating an inner vinylidene chloride copolymer layer attached on both sides to a layer of vinyl chloride polymer through the intermediacy of a polymeric layer, which have an improved delamination resistance which does not deteriorate after a period of residence in boiling water.
To this end, the invention provides rigid and semirigid thermoplastic structures with multiple coextruded polymeric layers incorporating an inner vinylidene chloride layer, each face of which is bonded to a layer of vinyl chloride polymers (2) having a gel temperature not exceeding 190.degree. C. and a dynamic viscosity at 180.degree. C. and 1 sec.sup.-1 not exceeding 2 10.sup.5 Pa s through the intermediacy of a separate intermediary layer of vinyl chloride polymer (1) whose gel temperature and dynamic viscosity measured at 180.degree..degree.C. and 1 sec.sup.-1 are lower than those of the adjacent vinyl chloride polymer (2).
DESCRIPTION OF THE PREFERRED EMBODIMENT
The thermoplastic structures with multiple coextruded polymeric layers according to the invention thus incorporate at least five layers, namely an inner vinylidene chloride copolymer layer provided on each of its two faces with two separate layers of vinyl chloride polymers (1) and (2). It will be understood that the vinyl chloride polymers (1) and (2) coextruded onto one face of the vinylidene chloride copolymer layer need not necessarily be identical to the vinyl chloride polymers (1) and (2) coextruded onto the opposite face of the vinylidene chloride copolymer layer. Nevertheless, preference is given to thermoplastic structures incorporating five coextruded layers in which the vinyl chloride polymers (1) and (2) are identical on both sides of the vinylidene chloride copolymer layer.
It is also to be understood that the multilayer structures according to the invention may incorporate additional layers such as, for example, layers consisting of polymeric scrap originating from recycling or, similarly, additional layers of thermoplastic polymers other than vinyl chloride polymers.
The invention thus lies substantially in resorting, as an intermediary layer between a vinylidene chloride copolymer layer and a coextruded layer of vinyl chloride polymer (2), to a vinyl chloride polymer (1) whose gel temperature and dynamic viscosity measured at 180.degree. C. and 1 sec.sup.-1 are lower than those of the adjacent vinyl chloride polymer (2). The layer of vinyl chloride polymer (1) acts both as an adhesion layer and as a thermal buffer for the vinylidene chloride copolymer.
Gel temperature and dynamic viscosity of the polymers forming the multilayer complexes according to the invention are understood to refer to the gel temperature and the dynamic viscosity of the compositions based on these polymers which are in fact employed for coextrusion, that is to say polymers containing all the components effectively employed in use, such as lubricants, plasticizers, heat stabilizers, fillers, pigments and the like. In the case where the polymers contain plasticizers, the quantity of plasticizer will not exceed 30% by weight and, preferably, 10% by weight.
The preferred multilayer structures are those in which the vinyl chloride polymer (1) forming the intermediary layer has a gel temperature not exceeding 180.degree. C. and a dynamic viscosity at 180.degree. C. and 1 sec.sup.-1 (.eta..sub.1) not exceeding 8 10.sup.4 Pa s.
Multilayer structures which are particularly preferred are those in which the vinyl chloride polymer (1) has a gel temperature not exceeding 170.degree. C. and a dynamic viscosity at 180.degree. C. and 1 sec.sup.-1 (.eta..sub.1) not exceeding 4 10.sup.4 Pa s.
Furthermore, it is particularly advantageous that the ratio of the dynamic viscosities of the vinylidene chloride copolymer at 170.degree. C. and 1 sec.sup.-1 (.eta..sub.0) and of the vinyl chloride polymer (1) forming the intermediary layer at 180.degree. C. and 1 sec.sup.-1 (.eta..sub.1) should be within the limits 0.05 .ltoreq..eta..sub.0 /.eta..sub.1 .ltoreq.4, preferably 0.05 .ltoreq..eta..sub.0 /.eta..sub.1 .ltoreq.2 and, still more particularly, 0.1 .ltoreq..eta..sub.0 /.eta..sub.1 .ltoreq.2.
The multilayer structures which are most especially preferred are consequently those in which the vinyl chloride polymer (1) forming the intermediary layer has a gel temperature not exceeding 170.degree. C. and a dynamic viscosity at 180.degree. C. and 1 sec.sup.-1 (.eta..sub.1) not exceeding 4 10.sup.4 Pa s, the vinyl chloride polymer (2) has a gel temperature not exceeding 190.degree. C. and a dynamic viscosity at 180.degree. C. and 1 sec.sup.-1 (.eta..sub.2) not exceeding 2 10.sup.5 Pa s, and the ratio of the dynamic viscosities .eta..sub.0 /.eta..sub.1 as defined above is between 0.1 and 2.
A vinyl chloride polymer is intended to refer to all the polymers containing at least 50% and, preferably, at least 70% by weight of monomer units derived from vinyl chloride. The vinyl chloride polymers which can be employed for the production of layers (1) and (2) of multilayer structures according to the invention thus include both vinyl chloride homopolymers and its copolymers containing monomer units derived from one or more comonomers and their mixtures. As non-restrictive examples of such vinyl chloride comonomers mention may be made of olefins such as ethylene, propylene and styrene, and esters such as vinyl acetate and alkyl acrylates and methacrylates. Preference is given to vinyl chloride homopolymers.
Vinylidene chloride copolymer is intended to refer to vinylidene chloride copolymers containing from 60 to 95% by weight of vinylidene chloride, the remainder consisting of one or more ethylenically unsaturated comonomers such as, for example, vinyl chloride, acrylic and methacrylic acids and esters, and acrylic and methacrylic nitriles. Nevertheless, preference is given to binary copolymers of vinylidene chloride and vinyl chloride containing approximately 75 to 85% by weight of vinylidene chloride.
To produce the multilayer structures according to the invention use may be made of the usual methods of coextrusion through a flat or round, feed-block or multi-manifold, die. These methods are characterised in that the streams of the molten polymers forming the various layers combine and travel together in the molten state before the exit of the single die. Nevertheless, preference is given to multilayer structures coextruded through a feed-block die and, still more particularly, a flat feed-block die.
The thickness of the polymeric layers forming the multilayer complexes according to the invention, and the total thickness of the said structures is not critical and depends, of course, on the use of which they are intended and on the degree of impermeability required. To give an idea, the total thickness of the thermoplastic multilayer structures is generally between 130 and 2,500 microns and, preferably, between 180 and 2,000 microns. The thickness of the central vinylidene chloride copolymer layer is, in general, between 10 and 850 microns and, preferably, between 35 and 500 microns, and that of the intermediary layers of vinyl chloride polymer (1) providing the adhesion between 1 and 100 microns, and preferably, between 2 and 50 microns. The multilayer structure according to the invention may consequently be in the form of films, sheets, plaques, preforms and rigid or semirigid hollow bodies. They can advantageously be employed for the manufacture of packages for alimentary, pharmaceutical and cosmetic products. Stiff multilayer structures with five polymeric layers coextruded through a flat feed-block die are especially suitable for the manufacture of of packages for pharmaceutical products in the form of pills or coated pills commonly referred to as "blister".
The examples which follow are intended to illustrate the invention without, however, restricting it.